The present invention relates to a laser scanning optical system including a reading optical system for reading information on an object surface and/or an image forming optical system for forming images on a photoconductive surface.
A conventional laser scanning optical system is provided with a light source such as a semiconductor laser, a light deflecting device such as a polygonal mirror, and a scanning lens system such as an f.theta. lens. A laser beam is emitted by a semiconductor laser, and is deflected by the polygonal mirror. Then the laser beam passes through the f.theta. lens to scan a predetermined area on an object surface (i.e., the main scanning is executed). While the main scanning is being executed, the object surface is moved in a direction orthogonal to the direction of the main scanning (i.e., the auxiliary scanning is executed). Thus, the object surface is two-dimensionally scanned.
The scanning optical system generally includes a plurality of lenses respectively having complex surfaces such as toric surfaces. Surface accuracies of such lenses are relatively difficult to achieve during manufacture. Thus, when they are used in the image reading optical system, chromatic aberration occurs.
Recently, in order to avoid occurrence of chromatic aberration, a reflection type optical system employing mirror(s) instead of lenses has been proposed. In an example of such an optical system, a beam emitted by a light source is deflected by a polygonal mirror. The deflected beam is directed towards a cylindrical mirror which functions as an f.theta. lens. Note that in this type of conventional arrangement, the beam emitted by the light source, the beam deflected by the polygonal mirror, and the beam reflected by the cylindrical mirror are on the same plane. Accordingly, a half mirror is inserted between the polygonal mirror and the cylindrical mirror, and the beam reflected by the cylindrical mirror is reflected by the half mirror and directed to a photosensitive surface to be scanned. Another example of such a system has substantially the same construction, but the cylindrical mirror is tilted so that the beam reflected by the cylindrical mirror passes out of the plane which includes the beam emitted by the light source and the beam deflected by the polygonal mirror.
When the optical scanning system employs the polygonal mirror as a deflection device, a deflection point at which an incident beam is deflected varies in the direction of the optical axis of the beam as the polygonal mirror rotates. Consequently, the incident beam of light is not a parallel beam, and a curvature of the image surface occurs. Further, if the cylindrical mirror is tilted as described above, the reflected beam forms a scanning line which is curved in the direction of the auxiliary scanning. In other words, a "bow" (i.e., a bend of a scanning line in the direction of the auxiliary scanning) occurs due to the tilted arrangement of the cylindrical mirror.
The curvature of the image surface is canceled by the shape of the surface of the cylindrical mirror and the shape of a cylindrical lens through which the reflected beam passes. With respect to the "bow", since the reflected beam passes through the cylindrical lens, which functions as a reducing optical system, the effect of the "bow" is suppressed.
In the former example, however, the beam deflected by the polygonal mirror and the beam reflected by the polygonal mirror are in the same plane (i.e., in a main scanning plane). In such a construction, since the light source should be positioned out of the area where the scanning is effective, the scannable angle (the angle within which the beam is capable of scanning) is limited to a relatively small angle. Further, since the curvature of the image surface is formed non-symmetrically with respect to the optical axis of the optical system, it is difficult to compensate for distortion or the like.
In the system described above, the "bow" is reduced, but is not canceled. Thus the scanning line remains bent although the bending amount is small, and the system cannot be applied to those situations requiring accurate imaging (i.e., high resolution imaging).